<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>float方式三栏布局</title>
  <style>
    .outer {
      width: 100%;
      min-width: 700px;
    }
    .side {
      height: 300px;
    }

    .left {
      float: left;
      width: 300px;
      background-color: lightblue;
    }
    .right {
      float: right;
      width: 300px;
      background-color: lightcoral;
    }

    .mid {
      float: left;
      width: calc(100% - 600px);
      background-color: lightcyan;
    }



  </style>
</head>
<body>

  <div class="outer">
    <div class="side left">left</div>
    <div class="side mid">mid</div>
    <div class="side right">right</div>
  </div>

  
</body>
</html>